sys: make stdout/stderr configurable during runtime

sys: make stdout/stderr configurable during runtime

Adds cvar sys_stdout which offers the same behaviours as the -nostdout
and -stderr cmdline options and is initialised by them if they're passed.
Makes -noterminal cmdline option a synonym for -nostdout, previously it
was effectively the same but less efficient.

Platforms with the fcntl() syscall (ie non-Windows):
Changes to non-blocking stdout/stderr by default to prevent stutters and
halts in certain scenarios.
Adds cvar sys_stdout_blocks for enabling blocking stdout/stderr if
required.

Retrieves stdio stream FDs instead of assuming they're always 0,1,2.

Signed-off-by: bones_was_here <bones_was_here@xonotic.au>